<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8" />
    <title>Document</title>
    <script src='lib/webgl-utils.js'></script>
    <script src='lib/webgl-debug.js'></script>
    <script src='lib/cuon-utils.js'></script>
</head>
<body onload="main()">
<canvas id="mybox" width="500" height="400">
    请更换新版浏览器查看效果!
</canvas>
<script >
   let VSHADER_SOURCE='attribute vec4 a_position;void main(){gl_Position=a_position;}'
   let FSHADER_SOURCE='void main(){gl_FragColor=vec4(1.0,0.0,0.0,1.0);}'

    function main(){
        let canvas=document.getElementById('mybox');
        let gl=getWebGLContext(canvas);
        if(!initShaders(gl,VSHADER_SOURCE,FSHADER_SOURCE)){
            console.log('初始化shader失败');
            return;
        }
        let n=initVertexBuffers(gl);
        if(n<0){
            console.log('设置顶点位置失败');
            return;
        }
        gl.clearColor(0.0,0.0,0.0,1.0);
        gl.clear(gl.COLOR_BUFFER_BIT);
        gl.drawArrays(gl.TRIANGLES,0,n);
        // gl.drawArrays(gl.LINE_LOOP,0,n);
        // gl.drawArrays(gl.LINE_STRIP,0,n);


    }

    function initVertexBuffers(gl){
        let vertices=new Float32Array([0.0, 0.5,   -0.5, -0.5,   0.5, -0.5]);
        let n=3;
        let vertexBuffer=gl.createBuffer();
        gl.bindBuffer(gl.ARRAY_BUFFER,vertexBuffer);
        gl.bufferData(gl.ARRAY_BUFFER,vertices,gl.STATIC_DRAW);
        let a_position=gl.getAttribLocation(gl.program,'a_position');
        if(a_position<0){
            console.log('a_position 不存在');
            return ;
        }
        // 将缓冲去变量分配给attribute变量
        gl.vertexAttribPointer(a_position,2,gl.FLOAT,false,0,0);
        // 开启attribute变量
        gl.enableVertexAttribArray(a_position);
        return n;


    }

</script>

</body>
</html>